High strength steel and method of making same

ABSTRACT

A thermally treated steel is provided comprising a steel having a microhardness of between approximately 60 and 67 HRC at a surface depth between approximately 0.002 to 0.014 inches from the steel surface and having approximately 0-36% by volume martensite converted from retained austenite at the surface depth. The steel may be used for ANSI 60 pins used in roller chain that may maintain elongation of less than 0.040 inches for up to 264 hours of wear testing. A method of making the steel pins is provided that comprises the steps of carburizing the steel in an atmosphere of 1.00% carbon or greater at a temperature between 1550° and 1750° F., quenching the steel to room temperature, tempering the steel to a temperature of 375° F. or less, cryogenically treating the steel to a temperature of −300° F. or lower and tempering the steel to a temperature of 375° F. or less.

BACKGROUND

The present invention relates generally to high strength steel and method of making high strength steel and, more particularly, to roller chain pins that are carburized and otherwise treated to allow for enhanced material hardness at depths below the material surface without the sacrifice of requisite metallurgical properties of the roller chain pin.

Typically, the pins, bushings and rollers, found in roller chains are carburized or case hardened. The carburizing process allows the outside of the parts to be transformed to a hard, wear resistant surface whereas the inner core retains the tough and ductile properties of the metal to absorb normal shock loading. In most applications, this combination provides the necessary engineered balance between wear resistance, durability and strength. In efforts to improve the overall performance of roller chain, including improvements in wear life, galling resistance and overall lubricity of the roller chain pins, additional thermal processes were sought.

Gas carburizing is a hardening process in which carbon is dissolved and diffused into the surface layers of a lower-carbon steel part at a temperature sufficient to render the steel austenitic. The austenitized steel is quenched in order to athermally transform the austenite to a martensitic phase. The resulting gradient in carbon content below the surface of the part causes a gradient in hardness, producing a strong, wear-resistant surface layer on a material.

SUMMARY OF THE INVENTION

The present invention provides for thermally treated steel comprising a steel having a microhardness of between 60 and 67 HRC at a surface depth between approximately .002 to .014 inches from the steel surface. In an embodiment, the steel may be used for ANSI 60 pins used in chain that may maintain elongation of less than 0.040 inches for up to 165 hours of wear testing. In an embodiment, the steel may be used for ANSI 60 pins used in chain that takes at least 200 hours of wear testing to elongate 0.070 inches or more. In an embodiment, the steel may be used for ANSI 60 pins used in chain that has at least 168% better wear characteristics than standard pins with respect to elongation over wear time. In an embodiment, the steel may form a pin provided in combination with bushings, link plates and roller bearings to provide roller chain.

In an embodiment, the method may further comprise the step of quenching the pin to room temperature. In an embodiment, the method may further comprise the step of quenching the pin using fast quench oil. In an embodiment, the method may further comprise the step of tempering the steel to a temperature of 375° F. or less.

In an embodiment, the carburizing step may provide for carburizing the pin in an atmosphere of 0.80% to 1.0% carbon or greater at a temperature of between 1550° and 1750° F. In an embodiment, the carburizing step may further comprise a carbonitriding process including an ammonia atmosphere. In an embodiment, the thermal treatment step may include deep cryogenic treatment of the pin at approximately −300° F. or lower. In an embodiment, the deep cryogenic treatment may occur for approximately 24 hours or less.

In an embodiment, the method may further comprise the steps of cryogenically treating the steel to a temperature of −300° F. or lower converting retained austenite to martensite and tempering the steel to a temperature of 375° F. In an embodiment, the steel may be tempered at approximately 375° F. for approximately 30 minutes or more prior to cryogenic treatment. In an embodiment, the steel may be tempered at approximately 375° F. after cryogenic treatment. In an embodiment, the method may further comprise the step of carbonitriding the steel. In an embodiment, the quenching step may occur in fast quench oil.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings an embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.

FIG. 1 is a perspective view of a chain of the present invention; and

FIG. 2 is a graph of Chain Wear Test Results of Thermally Treated ANSI 60 Pins.

DETAILED DESCRIPTION

The present invention provides for high strength steel and methods of making same. Such steel may be used for many applications such as for machinery components such as arbors, ball and roller bearings, gages, cylinders, pistons and raceways. In addition, the high strength steel may be used for chain used for machinery. For example, a roller chain 10 such as depicted in FIG. 1 includes pin link plates 12, 14 that pivotally connect pairs of pins 16. In an embodiment ANSI 60 pins may be used. Each pin 16 has a cylindrical bushing 18 and a roller bearing 20. Generally, the majority of the wear on the chain 10 affects the pins 16 and most of the rest of the wear affects the bushings 18. It is to be understood that the discussion below regarding the thermal treatment process of the present invention is with regard to individual pins 16, prior to assembly with the chain 10 as shown in FIG. 1.

Acceptable wear properties for such chain 10 are well defined. A chain is considered worn out when the overall length of the chain reaches 3% elongation. Such elongation is usually due to pin 16 and bushing 18 wear. An ANSI 60 pin is considered worn out when it wears to a depth of 0.017 inches from its original surface. As discussed below, the chain 10 of the present invention extends the wear life of ANSI 60 pins by at least 168% with respect to wear life over time, by use of the high strength steel of the present invention. The strengthening of the pins 16 of the present invention occurs by case hardening of the outer surface of the pin 16. In order to provide pins 16 that are not too brittle, the core hardness of standard ANSI 60 pins of approximately 40-50 HRC (Hardness Rockwell C Scale) is maintained while elevating the hardness at the surface. For example, hardness ratings are increased between the surface of the pin 16 to a depth of 0.028 inches in order to enhance wear properties. Although some tensile strength is sacrificed at the surface, due to the increased hardness, for applications such as roller chain 10, this will not detract from longer wear life properties.

In the present invention, increased hardness at the surface of the pins 16 is obtained by developing higher carbon concentrations which is fully transformed to a martensitic microstructure. Because martensite can be converted from formation of retained austenite, the present invention also concerns obtaining higher levels of carbon within austenite at the surface of the pins 16. Generally, the higher levels of carbon within austenite are obtained via the carburizing process. The present invention provides for a carburizing process having high concentrations of carbon. Treatment with elevated carbon creates additional retained austenite on the surface of the pins 16. As well, the ability to convert higher levels of retained austenite to martensite is obtained via deep cryogenic treatment. The combination of the carburizing, quenching, cooling and tempering treatments provide for steel having high strength, and improved wear resistance. Although the discussion below is with respect to steel in the form of pins 16, the present invention pertains to the treatment of steel provided in any form. As well, such a thermal treatment process may also be used for other components of the chain 10, such as the bushings 18.

As depicted below in the chart Example I, in an embodiment, pins 16 formed of AISI 1524 steel underwent a thermal treatment process of the present invention beginning with heating to an austenizing temperature of 1700° F. The furnace used took about 30 minutes to attain 1700° F. The gas carburizing process occurred by the introduction of carbon gas into the furnace (with natural gas or propane). In an embodiment, 1.10% C atmosphere in the furnace was maintained at 1700° F. for 110 minutes. The atmosphere may vary by plus or minus 0.1% C. The carbon concentration within the furnace can be monitored by carbon probes, 3-gas analyzer or shim analysis.

The pins 16 were then removed from the furnace and quenched in oil in order to form martensite. In an embodiment, the quenching occurred in oil at 100-150° F. for 30 minutes. In an embodiment, fast quench oil having 8-14 I.V.F. is used for accelerated cooling. The pins 16 were then allowed to cool to room temperature for approximately 60 minutes. The pins 16 were then returned to the furnace for tempering in order to reduce some of the brittleness of the pins 16. In an embodiment, the pre-cryogenic tempering occurred at 350° F. for 60 minutes.

The pins 16 were then cooled to room temperature and prepared for deep cryogenic treatment. For example, the cryogenic treatment equipment may be off-site, and the pins must be packed for shipment to the cryogenic treatment site. In an embodiment, liquid nitrogen may be used for the deep cryogenic treatment. Cryogenic treatment, such as provided by Cryocon, Inc. of Ogden, Utah, was provided. It is known that cryogenic treatment (soaking) of steel at approximately −300° F. or lower for about 24 hours acts to transform retained austenite to martensite. Following cryogenic treatment, the pins 16 were then allowed to warm to room temperature in order to minimize cracking. Finally, a post-cryogenic tempering treatment was provided to the pins 16. It is known that such tempering occurs at approximately 350° F. for about 60 minutes in order to relieve residual stress, temper the newly formed martensite and reduce brittleness. Double and triple tempering processes are known to improve impact resistance. The pins 16 were then cooled to room temperature, machined and assembled with the other components of the chain 10 as shown in FIG. 1. The chart below summarizes the above described thermal treatment cycle:

EXAMPLE I

Thermal Treatment Cycle for AISI 1524 Pins Phase Deep Post Cryogenic Heat-Up Carburizing Quenching Tempering Cryogenic −300° F. Tempering 1700° F. 1700° F. 100-150° F. 350° F. or Lower 350° F. Time 30 minutes 1.10% ± .1 C. 30 minutes 60 minutes 24 hours 60 minutes 110 minutes

Example I provides for carburizing in an atmosphere of 1.10% C for 110 minutes at 1700° F. The thermal treatment process results in pins 16 that have higher concentrations of martensite at surface depths down to at least 0.018 inches. Photomicrographs of the sample pins 16 that were treated according to the above thermal treatment cycle of Example I, depict 24-30% retained austenite at the surface of the pins 16. The retained austenite is generally white prior to cryogenic treatment but is fully transformed to martensite after cryogenic treatment. The martensite appears black or grayish and is needle shaped (acicular) and has plates or lath shaped particles. Such pins treated by the thermal treatment process of Example I provided for a wear life increase of 279-383% over a baseline wear life for standard chain. Example I demonstrates that the high carbon atmosphere during carburization and the other treatment steps lead to higher levels of retained austenite and in turn higher microhardness results after the complete thermal treatment cycle as follows: TABLE I EXAMPLE I MICROHARDNESS (HRC) Depth #1 #2 #3 Average 0.002 65.4 64.0 63.7 64.4 0.004 64.9 63.9 63.6 64.1 0.006 64.9 62.6 63.9 63.8 0.008 64.6 62.2 62.7 63.2 0.010 62.5 60.9 60.9 61.4 0.012 61.3 60.8 59.8 60.6 0.014 60.3 60.1 58.7 59.7 0.016 58.8 57.6 57.8 58.1 0.018 57.2 57.0 55.4 56.5 0.020 55.8 54.7 54.9 55.1 0.022 54.3 53.4 53.0 53.6 0.024 53.6 52.7 52.0 52.8 0.026 52.5 51.8 51.5 51.9 0.028 51.8 51.1 50.8 51.2 0.119 49.1 49.5 49.1 49.2 average 58.5 57.5 57.2 57.7 max 65.4 64.0 63.9 64.4 min 49.1 49.5 49.1 49.2

Table I depicts microhardness results for three pin samples treated according to Example I thermal treatment cycle. As shown in Table I a microhardness as high as 65.4 HRC was achieved for Sample 1 at a depth of 0.002 inches. Applicant believes that the results of Table I show fairly uniform readings for the thermal treatment process provided by Example I. Variation in results between samples 1, 2 and 3 may be attributed to minor variations in temperature settings of equipment across the multiple thermal treatment process. Variation in results may also occur due to conditions during carburization, furnace conditions and location of each sample within the furnace. The variation in results between these three samples, for example at 0.006 inches of 2.3 HRC indicates that under certain conditions hardness readings may have ±2.5 HRC variation.

It is noted that the depth of 0.119 inches is the core of the pin and results at this depth demonstrate that the core hardness is approximately 25% less than the hardness levels at the surface.

As well, the above testing indicated microhardness measurements of Example I prior to cryogenic treatment and conversion of retained austenite to martensite of 60.6, 63.2 and 63.5 average HRC at 0.002, 0.004 and 0.006 inches, respectively. Comparison of the hardness results of the pins 16 of the present invention compared with standard pins shows that the pins 16 are at least 7% harder than standard pins at the surface.

The pins 16 of Example I were assembled in chain 10 and tested for wear. FIG. 2 is a graph of Chain Wear Test Results of Thermally Treated ANSI 60 Pins. The test was an accelerated wear test whereby a length of chain was run on two hardened steel sprockets in a horizontal layout at a specified load and speed in excess of those expected in application. The chain was tested with the initial application of lubrication only (ILO) and periodically removed from the drive and measured in two equal sections for chain length. The testing continued until the chain reached a specified amount of elongation. Test conditions were as follows:

76 pitches in length

23 teeth at 1200 revolutions per minute on both sprockets

100 lbs. of load applied

These conditions result in 5.2 horsepower being transmitted to the test chain

The test ran until the elongation reached 0.002 inch per pitch

FIG. 2 illustrates the wear results and the benefit of the present thermal treatment method. FIG. 2 depicts chain wear results of a section of chain having pins treated as shown in Example I above. The line on the graph designated by circles is the result of wear testing of an elevated probe after heat treatment and thermal treatment of the pins as described for Example I above. The line on the graph designated by squares is the result of wear testing of a standard probe heat treatment after thermal treatment of the pins. The line on the graph designated by triangles provides a baseline for chain wear comparison. The horizontal dashed line across the graph at approximately 0.074 inches elongation represents the point where each chain pitch has an average of 0.002 inch/pitch wear. FIG. 2 demonstrates that chain having pins 16 treated as shown in Example I takes approximately 300 hours to reach 0.07 inch elongation compared to 80 hours for standard chain. The additional 220 hours of wear life is more than a 275% improvement for the thermally treated chain pins of the present invention. The testing also shows that the chain having the treated pins 16 maintain elongation of less than 0.040 inches after 250 hours of wear testing.

EXAMPLE II

Thermal Treatment Cycle for AISI 1524 Pins Phase Deep Post Cryogenic Heat-Up Carburizing Quenching Tempering Cryogenic −300° F. Tempering 1700° F. 1700° F. 100-150° F. 350° F. or Lower 350° F. Time 30 minutes .85% ± .1 C. 30 minutes 60 minutes 24 hours 60 minutes 110 minutes

Example II depicts a thermal treatment cycle similar to Example I, except that the carburizing process has been modified. The pins were carburized in an atmosphere of 0.85%±0.1 C for 110 minutes. Photomicrographs of the sample pins 16 that were treated according to the above thermal treatment cycle of Example II depict less than 10% retained austenite at the surface of the pins 16.

Comparing Example I and II below, it can be seen that the increased carbon atmosphere leads to higher levels of retained austenite and in turn higher microhardness results for the pins treated according to Example I. Hardness testing for the pins 16 after the complete thermal treatment cycle according to Example II provided the following results: TABLE II EXAMPLE I MICROHARDNESS RESULTS (HRC) Depth #1 #2 Average 0.002 60.2 59.7 60.0 0.004 60.7 60.6 60.7 0.006 60.9 60.6 60.8 0.008 60.7 60.1 60.4 0.010 58.7 59.2 59.0 0.012 58.8 58.9 58.9 0.014 57.4 57.4 57.4 0.016 56.5 55.7 56.1 0.018 55.1 55.5 55.3 0.020 54.2 54.8 54.5 0.022 53.3 54.3 53.8 0.024 52.1 53.0 52.6 0.026 51.7 51.5 51.6 0.028 50.2 51.7 51.0 0.119 50.1 51.1 50.6 Average 56.0 56.3 56.2 Max 60.9 60.6 60.8 Min 50.1 51.1 50.6

Thus, it can be seen in the chart above that the hardness readings are lower in comparison to the HRC readings of Table I. Thus, comparing Example I and Example II it is demonstrated that the higher carbon enriched atmosphere during the carburizing process can cause the resulting steel to have a higher hardness measurement at certain depths. The variation in the average microhardness for each depth shows an increase of between 1.7 to 4.4 HRC at depths from 0.002 to 0.014 inches between Examples I and II.

As well, the surface hardness is approximately 25% higher than the core hardness of such pins 16. Measurements for core hardness of the pins of Examples I-IV provided approximately 47-52 HRC at a depth of 0.119 inches. Such a contrast between the core and surface hardness is favorable because such pins 16 have resiliency and toughness at the core, but have great strength and increased wear resistance at the outer surface. Chain using pins of Example II had a wear life increase of 168-180% over baseline wear life for standard pins. Chain having pins treated according to Example II reached 0.071 inch elongation after wear testing for 212 hours and attained 0.038 inch elongation at 164 hours.

In addition, microhardness testing of Example II prior to cryogenic treatment results in measurements of 59.9, 60.7 and 60.6 average HRC at 0.002, 0.004 and 0.006, respectively.

EXAMPLE III

Thermal Treatment Cycle for AISI 1524 Pins Phase Carburizing Deep Post Cryogenic Heat-Up 1600° F. with Quenching Tempering Cryogenic −300° F. Tempering 1700° F. Ammonia 100-150° F. 350° F. or Lower 350° F. Time 30 minutes 1.00% ± .1 C. 30 minutes 60 minutes 24 hours 60 minutes 110 minutes

Example III differs from Examples I and II with regard to the atmosphere of carburizing process. Carbonitriding is a modified process of carburizing in which ammonia is added to the carbon enriched gas. Example III provides for a carbonitriding in an atmosphere of 1.00% C±0.1 C for 110 minutes at 1600° F. Such pins treated by the thermal treatment process of Example III provided for retained austenite prior to cryogenic treatment of 20-24% which was athermally transformed to martensite after cryogenic treatment. Comparing Example III and IV (also carbontriding) below, it can be seen that the increased carbon atmosphere leads to higher levels of retained austenite and in turn higher microhardness results for the pins treated according to Example III as follows: TABLE III EXAMPLE III MICROHARDNESS Depth #1 #2 #3 Average 0.002 65.2 63.7 64.0 64.3 0.004 63.0 63.0 64.4 63.5 0.006 62.0 61.6 64.8 62.8 0.008 61.5 60.4 63.7 61.9 0.010 60.7 58.6 62.3 60.5 0.012 58.9 57.2 60.2 58.8 0.014 56.4 55.5 59.3 57.1 0.016 53.9 53.0 58.7 55.2 0.018 53.8 52.8 55.0 53.9 0.020 52.9 52.4 54.5 53.3 0.022 52.5 52.0 52.9 52.5 0.024 51.9 51.3 52.1 51.8 0.026 52.3 51.5 51.8 51.9 0.028 52.2 51.4 51.6 51.7 0.119 50.6 50.2 50.2 50.3 average 56.5 55.6 57.7 56.6 max 65.2 63.7 64.8 64.3 min 50.6 50.2 50.2 50.3

Table III depicts the microhardness results of pins treated by the thermal treatment process of Example III. Such pins provided for chain wear life increase of 237-344% over baseline wear life for chain having standard pins.

In addition, microhardness testing of Example III prior to cryogenic treatment provided results of 62.3, 63.1 and 63.4 average HRC at 0.002, 0.004 and 0.006 inches, respectively.

EXAMPLE IV

Thermal Treatment Cycle for AISI 1524 Pins Phase Carburizing Quenching Deep Post Cryogenic Heat 1600° F. with 100-150° Tempering Cryogenic −300° F. Tempering 1700° F. Ammonia F. 325° F. or Lower 350° F. Time 30 minutes .80% ± .1 C. 30 minutes 60 minutes 24 hours 60 minutes 110 minutes

Example IV differs from Example III because the pins were treated by carbonitriding by use of ammonia in the atmosphere at 1600° F. at 0.80% C for 110 minutes. Such pins treated by the thermal treatment process of Example IV provided for retained austenite prior to cryogenic treatment of 16-20% which was athermally transformed to martensite during cryogenic treatment and microhardness results as follows: TABLE IV EXAMPLE II MICROHARDNESS Depth #1 #2 #3 Average 0.002 62.1 61.7 61.0 61.6 0.004 61.5 61.0 62.1 61.5 0.006 60.5 60.5 59.8 60.3 0.008 59.3 59.2 58.9 59.1 0.010 58.6 57.1 58.0 57.9 0.012 56.9 55.5 55.2 55.9 0.014 54.8 53.9 53.0 53.9 0.016 52.8 53.9 53.1 53.3 0.018 51.9 52.5 51.5 52.0 0.020 51.1 51.0 51.3 51.1 0.022 50.4 50.2 50.5 50.4 0.024 50.0 50.4 50.4 50.3 0.026 50.0 50.4 50.0 50.1 0.028 50.2 50.4 50.4 50.3 0.119 47.9 49.1 50.1 49.0 average 54.5 54.5 54.4 54.4 max 62.1 61.7 62.1 61.6 min 47.9 49.1 50.0 49.0

Comparing Table IV above with Table III demonstrates that the higher carbon atmosphere of 1.00% C for Example III contributed to higher microhardness results for all samples at all depths above 0.016 inches than Example III which provided a 0.80%±0.1C atmosphere. The variation in the average microhardness for each depth shows an increase of between 1.9 to 3.2 HRC at depths from 0.002 to 0.014 inches between Examples III and IV.

In addition, microhardness results for Example IV prior to cryogenic treatment were 61.4, 61.8 and 61.6 average HRC at 0.002, 0.004 and 0.006 inches, respectively.

The above results were obtained for pins 16 treated as depicted the above Examples I through IV. However, the present invention provides for thermal treatment having other parameters. For example, the initial heat-up phase can extend between 5 minutes to 60 minutes to achieve between 1550° and 1750° F. The carburizing phase can include treatment between 1550° and 1750° F. extending between 15 minutes or more depending on carburizing depth having 0.85% to 1.40% carbon enriched atmosphere. The quenching phase can include treatment between 60° and 150° F. The tempering phases may include treatment between 100° and 375° F. for a minimum of 30 minutes. The deep cryogenic phase may include treatment between 0° F. and −350° F. extending between 12 to 48 hours and post cryogenic tempering between 100° and 375° F. for a minimum of 30 minutes. Such ranges of treatment are expected to provide similar ranges of hardness results as provided above with variation in microhardness results of ±2.5 HRC as discussed above. It is possible that microhardness results of 70 HRC or higher may be obtained using the above thermal treatment process under ideal circumstances.

While particular embodiments of the present application have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the principles of the present application in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the present application. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the present application is intended to be defined in the following claims when viewed in their proper perspective based on the prior art. 

1. A thermally treated steel comprising: a steel having a microhardness of between approximately 60 and 67 HRC from the surface to a depth of approximately 6% of the part thickness (or diameter).
 2. The steel of claim 1 wherein the steel is used for pins used in ANSI 6-roller chain that has elongation of less than 0.040 inches for up to approximately 264 hours of wear testing.
 3. The steel of claim 1 wherein the steel is used for pins used in ANSI 60 roller chain that takes at least 302 hours of wear testing to elongate 0.070 inches or more.
 4. The steel of claim 1 wherein the steel is used for pins used in ANSI 60 roller chain that has at least 279% better wear characteristics than standard pins with respect to elongation over wear time.
 5. The steel of claim 1 wherein the steel is used for pins used in ANSI 60 roller chain that has a microhardness of approximately 65.4 HRC at 0.002 inches from the surface.
 6. The steel of claim 1 wherein the steel is used for pins used in ANSI 60 roller chain that has a microhardness of approximately 60.3 HRC at 0.014 inches from the surface.
 7. The steel of claim 1 where the core hardness is 25% less than the surface hardness.
 8. The steel of claim 1 where the steel forms a pin provided in combination with bushings, link plates and roller bearings to provide roller chain.
 9. A method of thermally treating a pin of an ANSI 60 roller chain, the method comprising the steps of: providing the pin of steel; carburizing the pin at a high carbon concentration; and thermally treating the pin in order to convert retained austenite to martensite and provide a microhardness of 60 HRC or greater at a depth of between approximately 0.002 to 0.014 inches from the pin surface.
 10. The method of claim 9 further comprising the step of quenching the pin to about 100° F.
 11. The method of claim 9 further comprising the step of quenching the pin using fast quench oil.
 12. The method of claim 9 further comprising the step of tempering the steel to a temperature of 375° F. or less.
 13. The method of claim 9 wherein the carburizing step provides for carburizing the pin in an atmosphere of 1.00% carbon or greater at a temperature of between 1550° and 1750° F.
 14. The method of claim 9 wherein the carburizing step may further comprises a carbonitriding process including an ammonia atmosphere.
 15. The method of claim 9 wherein the thermal treatment step includes a deep cryogenic treatment of the pin at approximately −300° F. or lower in order to convert retained austenite to martensite.
 16. The method of claim 15 wherein the deep cryogenic treatment occurs for approximately 24 hours.
 17. A thermally treated steel comprising: a thermally treated steel that prior to cryogenic treatment has a microhardness of between approximately 55 and 65 HRC from the surface to a depth of approximately 6% of the part thickness (or diameter) and having approximately 15-50% by volume retained austenite near the surface.
 18. The steel of claim 17 wherein after cryogenic treatment the steel has a microhardness of between approximately 60 and 67 HRC from the surface to a depth of approximately 6% of the part thickness (or diameter) and having full transformation of retained austenite to martensite near the surface.
 19. The steel of claim 18 wherein the hardness of the steel at the surface is at least 25% harder than at a core of the steel.
 20. The steel of claim 18 wherein the steel provides a component having a core and a case over the core with the case being harder than the core and the case having at least approximately 15% martensite converted from retained austenite during thermal treatment. 